Recently, much progress has been made in increasing the brightness of light-emitting diodes (LEDs). As a result, LEDs have become sufficiently bright and inexpensive to serve as light sources in for example general lighting applications.
Commercial high brightness LED-technologies employs so-called thin-film technologies for constructing semiconductors. The materials used for producing high brightness LEDs are selected from the alloys of group III and group V elements in the periodic table. The most widely used semiconductor is Gallium Nitride (GaN) that has a direct band-gap of 3.26 eV.
However, due to disadvantages associated with thin-film technologies, present semiconductor technology seeks to utilize the advantages of nanowires in the areas of LEDs and laser diodes (LDs), where the nanowires are used as active components. This is due to arising advantages in performance that is possible in nanostructures, such as light extraction at low defect concentration. The diameter and length of nanowires, depending on the growth conditions, can vary from a few nanometers to a few micrometers.
US 2005/0194598 discloses a light emitting diode having a nanowire structure. The LED employs GaN nanowires in which a n-type GaN nanowire, an InGaN quantum well and a p-type GaN nanowire are subsequently formed in a longitudinal direction by inserting the InGaN quantum well into a p-n junction interface of the p-n junction GaN nanowire. A plurality of such GaN nanowires are arranged in an array. FIG. 1 illustrates the electroluminescence (EL) spectrum of the LED when a direct current of 20 to 100 mA is applied thereto. As can be seen in the graph, the LED is a narrow banded blue light emitting diode with a peak wavelength of approximately 465 nm.
Other wavelengths, e.g. white light, may be obtained by adding a fluorescent material to a transparent insulating material filled among the nanowires, or by modifying the band-gap using alloys selected from the III-V group, such as AlInGaN. However, both the described methods for modifying the light emitting diode to obtain white light involve complicated and expensive manufacturing steps.
It is therefore an object of the present invention to provide an improved light emitting diode arranged to emit light having a broadband spectral distribution, which substantially overcomes the disadvantages of the prior art while providing further improvements in terms of cost and manufacturing convenience.